1. Field of the Invention
The present invention relates to the preparation of hydrosoluble and/or hydrodispersible polyesters and to the use of such polyesters for the sizing of textile threads, notably polyester threads.
By the term "threads" are intended both assemblies of long unitary filaments or yarns formed from discontinuous fibers.
2. Description of the Prior Art
Copolyesters which are soluble or dispersible in water, simultaneously comprising terephthalic, isophthalic, sodium-5-oxysulfonyl isophthalic, ethylene glycol and polyethylene glycol recurring structural units are described in FR-A-1,602,002. These are prepared via the copolycondensation of dimethyl terephthalate, dimethyl isophthalate, sodium-5-oxysulfonyl isophthalic acid and a mixture of ethylene glycol and polyethylene glycols, in which the polyethylene glycols constitute at least 20% of the mixture. Such copolyesters are used as temporary finishes in the textile industry. The process for the preparation thereof entails the use of relatively expensive diols, such as diethylene glycol.
In JP-A-88/256,619 it is indicated that the aforesaid process does not readily provide an adequate degree of polycondensation. To obviate this disadvantage, JP-A-88/256,619 describes first conducting a direct esterification stage between the terephthalic acid, the isophthalic acid and the ethylene glycol, followed by a transesterification reaction between the dimethyl sodium-5-oxysulfonyl isophthalate and an additional amount of ethylene glycol, and thence a final polycondensation. This process makes it possible to provide a minimum proportion, below 2%, of diethylene glycol units.